r-如何忽略 0 的行

示例数据集：

tibble::tribble(
     ~Item,        ~Date, ~quantity,
     "saw", "01/01/2020",        0L,
     "saw", "02/01/2020",       10L,
     "saw", "03/01/2020",        5L,
     "saw", "04/01/2020",       20L,
     "saw", "05/01/2020",       30L,
     "saw", "06/01/2020",        0L,
     "saw", "07/01/2020",        0L,
  "hammer", "08/01/2020",       60L,
  "hammer", "09/01/2020",       70L,
  "hammer", "10/01/2020",       40L
  )

我想从每种材料的最大日期中减去最小日期中的数量，因此它忽略具有 0 的日期

期望的结果：

tibble::tribble(
  ~Item, ~difference,
  "saw",        -20L,
  "hammer",       20L
  )